Manufacture of sodium sulphate



ug. 3, 1937. R. B. MacMuLLlN I MANUFACTURE OF SODIUM SULPHATE Filed Oct. 17, 1934 Patented Aug. 3, 1937 Ul'l'ED STA'Eq ATENT MANUFACTURE F SODIUM SULPHATE Robert Burns MacMullin, Niagara Falls, N. Y., assigner to The Mathieson Alkali Works, Inc., New York, N. Y., a corporation of Virginia Application October 17, 1934, Serial No. 748,607

2 Claims. (Cl. .Z3-121),

This invention relates to improvements in the precipitate and contaminate the product, sodium manufacture of sodium sulphate, and particularly sulphate. This invention includes as a feature anhydrous sodium sulphate, from sodium bicarthe return of all or part of the mother liquor, bonate and calcium sulphate. Sodium bicarbonat that stage of concentration at which sodium ate and calcium sulphate react, in aqueous media, carbonate just begins to precipitate in the form 5 to form sodium sulphate with precipitation of of Burkeite, 2Na2SO4.Na2CO3, to the reaction Vescalcium carbonate and liberation of carbon disel. Sodium carbonate present in the mother oxide. The reaction may be typified as follows: liquor returned to the reaction vessel is then con- 2NaHCO3+CaSO4Na2SO4+CaCO3+CO2 +H2O verted to sodium sulphate. The reaction may be typified as follows:

Following separation of the precipitated calcium carbonate, the sodium sulphate can be recovered Cas04+NazCOs-)CaCOH-Nazs from the resultant liquor, as anhydrous sodium Crude ammonia soda, as ordinarily formed, consulphate for example by evaporation at temperatains NaCl, Na2SO4, NazCOa, NH4HCO3, CaCOa,

tures upwards of 33 C. This invention relates, MgCOs and SiOz as impurities. Of these impuri- 15 more particularly, to an improved cyclic op'eraties, sodium chloride is the only one which collects tion for carrying out this reaction. and eventually has to be removed either with the In carrying out this invention, sodium bicar- Na2SO4 produced or by purging the system of a l bonate, in the form of crude ammonia soda, and small quantity of the mother liquor remaining 2o calcium sulphate are introduced into a reaction after separation of the NazSOi produced. The 20 Vessel, in an aqueous reaction medium, in which sodium sulphate is a useful impurity. NazCOs the reaction forming sodium sulphate is compresent as an impurity, is converted into Na2SO4. pleted. The liberated carbon dioxide and the NH4HCO3 present is decomposed in the reaction` liquor including dissolved sodium sulphate and vessel yielding NHa-l-COZ, both of which gases are suspended calcium carbonate are separately disrecoverable either from the reaction vessel or 25 charged from the reaction vessel. Calcium carfrom the vapors from the evaporator. The rebonate is separated from this liquor, by filtration, maining impurities CaCO3, MgCO3 and S102 are for example. Following separation of the calinsoluble and are removed along with the CaCOa cium carbonate, the liquor is concentrated, by produced by the reaction by filtration, for eX- evaporation, for example, to precipitate sodium ample. It will be apparent that when the in- 30 sulphate which is then separated from the convention is practiced using pure sodium bicarcentrated liquor. bonate in place of crude ammonia soda,y difficul- In this operation, in accordance with this inties ordinarily encountered because of the presvention, the calcium carbonate as initially sepence of impurities are eliminated and all the arated from the liquor discharged from the remother liquor remaining after separation of the 35 action vessel is Washed with sufficient water to sodium sulphate formed may be returned to the form, as wash liquor, the aqueous medium in reaction vessel without accumulation and evenwhich the sodium bicarbonate and calcium sultual contamination of the sodium sulphate prodphate are supplied to the reaction vessel, and the uct with sodium chloride.

concentrated liquor, following separation of pre- The amount of mother liquor returned to the 40 cipitated sodium sulphate, is returned to the rereaction vessel will depend largely on the conaction vessel. Important economies are thus dition of evaporation, i. e., the temperature and secured. pressure employed, which determine the solubil- In this connection it should be noted that the ity of NazCOa in the mother liquor. In general,

reaction between calcium sulphate and sodium the higher the temperature, the lower the solu- 45 bicarbonate approaches but never quite reaches bility of Na2CO3 in the Na2SO4 solution, and the equilibrium and accordingly the reaction liquor greater the proportion of mother liquor to evapwill contain some sodium carbonate. Upon evaporator feed, returned to the reaction vessel. oration sodium sulphate will iirst precipitate, Ihe invention will be further described in conthe sodium carbonate and other soluble impurinection with the accompanying drawing which 50 ties in the crude ammonia soda, such as sodium illustrates, schematically and conventionally, and

chloride, for example, concentrating in the somewhat as a flow sheet, an arrangement of mother liquor. Complete evaporation of the apparatus adapted for carrying out the invenmother liquor should be avoided as if to much o-f tion.

the mother liquor is evaporated, impurities will Referring to the accompanying drawing, the

trated liquor may be subjected to the highest apparatus illustrated comprises mixers I and 2, a reaction vessel 3, a lter 4, an evaporator 5, a separator 6, a centrifuge 1, a condensing column 8, storage vessels 9, I0 and I I, and vacuum pumps I2 and I3. Two or more reaction vessels in which the reaction is progressively completed may be substituted for the single reaction vessel 3 illustrated. The filter 4, for example, may be of conventional continuous rotary type, the discharge connections I1 and 29, connecting with a conventional automatic valve 36. Two, three or more iilters may be operated in parallel in the place of the filter 4, illustrated. The evaporator 5 illustrated may comprise tWoor more multiple effect evaporators With parallel viiovv of the sodiumA sulphate liquor therethrough or'it may comprise a series of multiple effect evaporators through Which the sodium sulphate liquor passes in succession, and in the latter case the most concentemperatureV With separation Aof precipitated sodium sulphate, as in separator 6, between each of the successive stages. The apparatus may also comprise a heat exchanger 39, a column still 31 and a condenser 38.

In carrying out the invention in the apparatus illustrated for example, sodium bicarbonate and calcium sulphate are introduced in the mixers I and 2, respectively, into a stream of the aqueous reaction medium supplied through connection I4 and introduced into the reaction vessel 3 through connection I5. Any carbon dioxide liberated in the mixer 2 may be discharged through connection 3I and separately recovered or discharged through connection 33 by means of connections 3I and 32,. respectively, and recovered With carbon dioxide liberated from the reaction vessel 3. In the reaction vessel 3, the reaction is completed at a temperature approximating 40-60 C., for example, the liberated carbon dioxidebeing discharged through connection 33. This liberated carbon dioxide may be discharged from the reaction vessel 3 under pressure if desired. The reaction mixture ilovvs from the reaction vessel 3 to the lter 4 Athrough connection I 6. After separation of calcium carbonate, the reaction liquor is discharged to storage Vessel II through connection I1. The separated calcium carbonate is Washed with Water, at a temperature approximating 50-60 C., for example, supplied from vessel 9 through connection I8 and the Washed calcium carbonate is discharged through connection I9. The Wash liquor is discharged through connection 20 to the storage vessel I9. Sodium sulphate is precipitated by concentration, in evaporator 5, of liquor supplied from storage vessel I I through connections 2| and 22. The concentrated liquor including precipitated sodium sulphate is discharged from the evaporator 5 to separator 6 through connection 23, the concentrated slurry being discharged from the separator 6 to centrifuge 1 through connection 24, the remaining concentrated liquor being discharged from the separator 6 through connection 25 or through connections 25 and 26. A further separation of liquor from the precipitated sodium sulphate is effected in the centrifuge 1, the separated liquor being returned to the evaporator 5 through connections 21 and 26 and the sodium sulphate being discharged through connection 28.

In the operation of the invention condensate from the steam chest of evaporator 5 may be discharged through connection 34 to vessel 9 and advantageously used as the'hot Wash Water for separated calcium carbonate. Vapors from evaporator 5 condensed in condensing column 8 may be passed to tank 29 and any ammonia or other gases present in the condensate recovered. Further, lovv pressure steam from one of the evaporating pans may be used to supply heat to promote the reaction in the vessel 3. This steam may be supplied, for example, through connection 39 to a heat exchanger 35 arranged in reaction vessel 3 or in any other suitable manner. This feature of the invention is advantageous in that it conserves steam, thereby promoting economy of operation. The liquor from vessel II may be supplied directly to evaporator 5, through connection40, or it may be passed through column still 31 on its Way to evaporator 5. In the column still 31, any ammonia and a large part of any free carbon dioxide present are stripped from the liquor, the distillation being effected by steam supplied through connection 4I, the distillate being recovered in condenser 38 discharging into any appropriate receiver, tank 29 for example. When such stripping is so effected, the liquor is with advantage preheated, in heat exchanger 39 for example, to minimize any dilution resulting from condensation of steam in still 31 tending to increase the burden on the evaporator 5. Steam `or hot condensate from the steam chest of evaporator 5, for example, may be used as the heating medium in heat exchanger 39. This feature of the invention is advantageous in that it facilitates the use of a barometric condenser as the condenser 8.

In accordance with this invention, concentrated liquor separated from the precipitated sodiumsulphate, in the separator -6 inthe apparatus illustrated, is returned, through connection 25 in the apparatus illustrated, to the reaction vessel, and Wash liquor from the storage vessel Ill'is used astheY aqueous medium, supplied through connection I4 in the apparatus illustrated, in Ywhich the sodium bicarbonate and calcium sulphate areY introduced into the reaction vessel.

The following example will further illustrate a continuous operation embodying the invention using apparatus of the general arrangement illustrated, in .which proportions are given in the form of rates, parts by Weight per unit of time:

About 218 parts of crude ammonia soda, containing about NaHCO3, and about 213 parts of anhydrite, containing about available CaSO4, both in a fine state of subdivision, are introduced into the reaction vessel in 210 parts of recirculated mother liquor containing 50 parts Na2SO4, 8 parts NazCOa, 8 parts NaCl, together With 550 parts of Weak Wash liquor containing 50 parts Na2SO4, 2 parts NazCOa, 2 parts NaCl, and With minor quantities of NH3 and CO2. In the reaction vessel the reaction is completed at a temperature of about 55 C. About 52 parts of carbon dioxide are liberated in the reaction vessel and about 5 parts in the lter. After separation of calcium carbonate, about 852 parts of liquor including about 249 parts NazSOr, 9 parts Na2CO3, 9 parts NaCl are discharged from the iilterV to the evaporator. The initially separated balcium carbonate is Washed with about 455 parts of hot water to form the 550 parts, approximately, of Weak Washvliquor which is part of the aqueous medium in which the reactants are supplied to the reaction vessel. After this Washing the separated calcium carbonate amounts to about 161 parts (dry basis) including about one part of soluble salts. After evaporation of about;

427 parts of water in the evaporator, over a temperature range approximating 77-101 C., about 197 parts of sodium sulphate, as Na2SO4, contaminated with about 12 parts of the concentrated liquor containing 3 parts NazSOi, 0.5 part NazCOa, 0.5 part NaCl, are recovered as the primary sodium sulphate product. About 210 parts of the concentrated liquor from which this sodium sulphate is separated, including about parts of sodium sulphate, 8.0 parts of sodium chloride and 8.0 parts of sodium carbonate are returned to the reaction vessel. Concentrated liquor separated from the sodium sulphate product in excess of this proportion is returned to the evaporator. The primary sodium sulphate product is dried to produce a final product containing 99.5% Na2SO4 or better.

With the added step of returning concentrated liquor from which sodium sulphate has been separated following concentration of the reaction mixture after separationY of precipitated calcium carbonate, as described herein, the reaction between sodium bicarbonate and calcium sulphate is with advantage carried out as described in an application iiled January 5, 1934, Serial Number 705,332.

I claim:

1. In the manufacture of sodium sulphate by reaction between sodium bicarbonate and calcium sulphate in an aqueous medium, the improvement which comprises introducing the sodium bicarbonate and calcium sulphate into a reaction vessel in an aqueous reaction medium, separating precipitated calcium carbonate from the reaction mixture discharged from the reaction vessel, washing the separated calcium carbonate With Water to form the aqueous medium in which the sodium bicarbonate and calcium sulphate are introduced into the reaction vessel, concentrating the reaction mixture by evaporation after separation of calcium carbonate to precipitate sodium sulphate, said evaporation being limited to prevent substantial precipitation of sodium carbonate, separating precipitated sodium sulphate from the concentrated liquor and returning concentrated liquor after separation of precipitated sodium sulphate to the reaction vessel.

2. In the manufacture of sodium sulphate by reaction between sodium bicarbonate and calcium sulphate in an aqueous medium, the improvement which comprises introducing sodium bicarbonate and calcium sulphate into a reaction vessel in an aqueous medium, separating precipitated calcium carbonate from the reaction mixture discharged from the reaction vessel, concentrating the reaction mixture by evaporation after separation of precipitated calcium carbonate to precipitate sodium sulphate, said evaporation being limited to prevent substantial precipitation of sodium carbonate and separating the precipitated sodium sulphate from the concentrated liquor, and returning the concentrated liquor after separation of sodium sulphate to the reaction vessel, the amount of sodiumv bicarbonate introduced being not substantially in excess of the molecular equivalent of the introduced calcium sulphate.

ROBERT BURNS MACMULLIN. 

